Considering the Cold War ended a long time ago I don't think there is anything against doing business with Russia. Now if he's sharing top secret information with Russia that could be a problem. Though I think Mueller would have found something by now, we have been spied on openly since the Patriot Act not much can remain a secret from the FBI especially if you are running for president.
Russian trolls may have happened but unless Russians actually hacked voting machines they didn't really do much. ...
russia did social media engineering on behalf of the trump campaign
if that was done with the knowledge/collusion of the trump campaign it's a crime
and some are taking falls even as we speak
allow it and the democrats will play the same, which means any/all other nation resources can be brought into the game on their side
fair is fair after all